1. Field of the Invention
The present invention relates to a gas mixture method for generating an ion beam, and more particularly to a method prolonging lifetime of an ion source for generating an ion beam.
2. Description of the Prior Art
Electronic products bring convenience to humans' life and all these products rely on controller chips for operation. Semiconductors have desired electrical properties and thus are widely used for manufacturing chips. Generally, semiconductor wafers need to be doped with chemical elements to reach different electrical properties for different uses. For example, n-type semiconductors can be doped with electron donors (e.g. Group V elements) to increase concentration of free electrons and it affords rise in conductivity.
Currently, ion implanting process is mainly adopted by industry for doping because it can depose ions onto wafer surface homogeneously. An ion implanter system comprises a gas source, a reaction chamber, magnets, and extraction electrodes. While the dopant gas is injected into the chamber, electrons emitted from the hot cathode collide with gas molecules to generate ions. Then magnets establish a magnet field to confine those electrons to prevent them from striking the reaction chamber before colliding with gas molecules, thus to increase electron density and the plasma density. Extraction electrodes placed adjacent to the aperture of the reaction chamber extract the ions out of the plasma to form an ion beam outside the chamber along a desired trajectory. Species of dopant gases contribute to electrical presence of doped wafers such as n-type semiconductors or p-type semiconductors and may comprise AsH3, GeF4, BF3, CO2, CO, PH3 etc. The reaction chamber which accommodates the anode, cathode, reactant gases is called ion source. Stable and uniform ion beam generated from the ion source is required. However, quality of the ion beam may degrade after operating in a period of time; the interval between operation failures is called mean time between failures (MTBF) or lifetime and is considered a critical factor regarding doping process. Larger MTBF value usually indicates better durability of the ion source.
Operation failure may result from several factors. For example, since plasma (ionized gas) is formed in the chamber, positive ions also can possibly accelerate to bombard the cathode to dissociate metal materials of the cathode, such as W (tungsten) or Mo (molybdenum), thereby etching away the cathode material to shorten cathode lifetime. However, if we add some fluorine containing gases, such as CF4, the halogen effect caused by fluorine ions can move W or Mo materials from the chamber wall back to the cathode surface (i.e. halogen cycle). As we can see, the mixture ratio between dopant gas and fluorine containing gas (one of minor gases) is critical to the competing processes of oxidation on W or Mo cathode surfaces versus W or Mo re-deposition on W or Mo cathode surface. Appropriate adjustment of the mixture ratio contributes to best performance and durability of the ion source.
Fluorine containing gases are used to prevent the cathode becoming thinner so as to prolong lifetime of the ion source. However, fixed mixture ratio of CF4 can possibly leads to a negative result. For example, CF4 gas is originally used to enhance halogen effect to maintain cathode size when generating an ion beam. Halogen cycle is a re-deposition process that metal materials evaporating from the cathode or chamber wall bind with halogen gases and return to the cathode. After processing in a period of time, the cathode starts to grow larger and requires higher power to emit electrons. Consequently, it accelerates erosion to the chamber wall due to ion bombardment with higher energy and shortens the lifetime of the ion source. Beneficial influence of adding CF4 is counteracted.
Therefore, a gas mixture method to properly control mixture ratio of the gas mixture needs to be developed to prolong lifetime of the ion source.